1. Field of the Invention
This invention relates generally to a motor vehicle driveline, particularly to a driveline in which primary wheels are connected continually to a power source and secondary wheels are connected selectively to the power source.
2. Description of the Prior Art
All-wheel-drive (AWD) systems tend to degrade vehicle fuel economy due to increased driveline parasitic losses even when AWD is not activated. These parasitic losses occur because some parts of the driveline continue to be driven by the engine and transmission, or the secondary drive wheels and their rotation cause a drag torque to be exerted on the driving element.
Driveline disconnect systems improve fuel economy by disconnecting as many of the driveline rotating parts as possible, as close to the transmission output and the secondary drive wheels as possible, when all-wheel-drive is not activated.
These disconnect systems provide a significant fuel economy benefit, but they pose challenges including (i) getting the driveline reconnected quickly when the AWD system must be activated, and (ii) maintaining system durability through many driveline disconnect/reconnect cycles. Meeting these challenges is complicated when some or all of the disconnect clutch or synchronizer designs are constrained by size and packaging limitations, or a desire to minimize drag. These constraints work against the characteristics required to provide fast and durable disconnect systems.
In the case of a front-wheel drive (FWD) based AWD vehicle, the initial phase of front disconnect and reengagement is performed using a synchronizer having a limited torque capacity, which limits the speed at which it is able to synchronize undriven drivetrain components elements by bringing them back to the same speed as the elements which are already rotating. The small size of the synchronizer also makes providing good durability under these circumstances a challenge.